Servomotor control apparatus for electric furnaces and the like



March 2, 1954 w. A. MOREY 2,671,181

SERVOMOTOR CONTROL APPARATUS FOR ELECTRIC FURNACES AND THE LIKE Filed Nov. 25, 1952 2 Sheets-Sheet 1 FIG.

FIG. 2

I N VEN TOR.

March 2, 1954 W. A. MOREY SERVOMOTOR CONTROL APPARATUS FOR ELECTRIC FURNACES AND THE LIKE Filed NOV. 25, 1952 2 Sheets-Sheet 2 INVENTOR.

, shown in Fig. 2;

Patented Mar. 2, 1954 I 'ssnvoMoroa CONTROL APPARATUS FOR ELECTRIC FURNACES AND THE LIKE Wendi-air A. Morey, Park Forest, 111., assignor to Whiting Corporation, a corporation of Illinois Application November 25, 1952, Serial No. 322,455

' 14 Claims. (01. 314-41.)

This. invention has reference to patentapplication, serial No. 299,294, iiled July 16, 1952, and refers to a hydraulic-electric control adapted to impart movement. to. a .servomotor proportional to a change inthe condition of an electrical circuit. It is particularly adapted-for use in regulating the electrode current' of a direct arc multiphase electric melting furnace ofthe type having movable electrodes. 7

Oneof the principal objects of this invention is to provide a sensitive, durable control which is low in' cost, easily maintainedjresistant to hunting and particularly adaptedfor use in connection with a novel type of. hydraulic servomechanism defined in said patent a pplicttion,. serial No. 299.294.

A further object cf-this; invention is the provision of a con'trolof thetyiie described capable ofoperating efilciently undervirtually all conditions.

Other objects, as wellaas many of the advantages of this invention, will {be described in the course of the following description and in the appended drawings, in which'z V Fig.1 1 illustrates diagrammatically the application of the subject 'inventionto a direct are electric melting furnace;

Fig. 2 is a schematic electrical diagram of the subject invention;

Fig. 3 illustrates a modification of the circuit Fig. 4 is a further diagrammaticillustration of modification of thearrangement-shown in Fig. 2, and

Fig. 5 is a still further modification of the arrangement shown in Fig. 2.

The subject controlin its preferred form employs a servomotor, usually an upright hydraulic cylinder, having a port communicatin with the act which are preierablyot the sh nt wound dinot current Wham mamasnunt 35 I motor, the speed wound direct current motors revolve at a constant speed. The speed of such motors may be increased by reducing the strength of the shunt field, or decreased by reducing the current flowing to the armature.

It will be noted that the cylinder, or servomotor, moves upwardly when the volume discharge or output of the first. pump exceeds the output of the second pump, and moves downwardly by gravity when such condition is reversed. If the servomotor operates to move an electrode arm and electrode, it will be obvious that the electrical energy in the air circuit varies inversely with the length of the arc and is therefore proportionate to the position of the electrode. Thus, by controlling properly the speed of one, or both, of the motors, the subject device may be made to maintain the current in the arc circuit at a substantial constant value. Such regulation maybe obtained by connecting a transformer or the like between the electrode circuit and ground, and the output employed to regulate the speed of one or both of the. pump motors. Similarly, a current transformer, inductively coupled to the electrode circult, may be employed for the same purpose, or the two may be combined.

For example, the output of the current transformer will be at a maximum when the arc length is at a minimum, and at the same time the output of the potential transformer, connected from the electrode circuit to ground, will be at a 'minimum. Under such conditions, the length of the arc should be increased. The output or signal from the current transformer may therefore be utilized to supply current to the armature of the first motor, thereby increasing its speed directly with the output, or to the field of the second of which will decrease with an increase of current, or to both simultaneously. In either event, the changes in speed will result in an upward movement of the electrode and an increase in the arc length. This action may, at

the option of the operator, be supplemented by connecting the secondary winding of the potenstrength of "the second motor field, or decreasing tical reciprocating movement which is connected with the current to the armature of the second motor, or both.

Fig. 1 illustrates a structure of the type described applied to a direct are electric melting furnace. A power transformer, generally designated ill, of conventional design is usually provided with inductance winding II in the transformer primary. Ordinarily, the transformer l will be of the three phase type, each of the three phases being connected to an electrode. For simplicity of illustration, only a single electrode is illustrated, it being understood that the other electrodes are substantial duplicates of that shown. The output current of the transformer I8 is delivered by rigid conductors l2 through a transformer room wall |3 to flexible conductors l4, which are in turn coupled to conductive members |6 connected to an electrode |1, supported on an electrode arm l8. The electrode arm l8 includes an upright member l9 guided for verby a plurality of rollers 2|. Within the member I9 is an upright hydraulic cylinder 22 having reciprocable ram 23 engaging the electrode arm l8 in such manner that extension of the ram 23 forces the arm l8, together with the electrode |1, upwardly.

The cylinder 22 is provided with a port or ports 24 near its bottom communicating with hydraulic conduits 26 and 21. The conduit 26, the conduit 21 either directly or through the cylinder 22, is connected to the outlet of a first positive displacement rotary hydraulic pump 28, suitably of the gear or vane type. The conduit 21 communicates with the inlet of a second pump 29, similar to the pump 28, but usually having a capacity approximately twice that of the pump 28. A conduit 3| connects an accumulator 32 with check valves 33 and 34, the former communicating with the inlet of the first pump 28, and the latter with the inlet of the second pump 29. A spring loaded check valve 36 connects the outlet of pump 28 with the accumulator 32, a similar valve 31 connecting the outlet of the pump 29 with the accumulator. Al

though not essential, I prefer to maintain in the accumulator 32 a quantity of air under pressure suificient to partially counterbalance the weight of the electrode arm I8 and the movable'parts associated therewith.

In operation, the pump 28 is continuously r driven in one direction only to supply hydraulic fluid to the cylinder 22. The second pumpf29 also operates continuously in a single direction to withdraw hydraulic fluid from the cylinder 22,

discharging back into the accumulator 32. Thus,

it will be apparent that if the discharge rate of the first pump 28 exceeds the discharge rate of the pump 29, the electrode arm l8 and the electrode l1 will be moved upwardly. Conversely, if the discharge rate of the second pump 29 'exceeds the discharge rate of the first pump 28', the electrode arm |8 will move downwardly by gravity.

The current flowing in, the secondary or are circuit, which includes the electrode l1, varies with the length of the arcing gap or are, which may be considered as the distance from the point of the electrode to a conductive charge 38 in the furnace shell 39. As the length of the arc increases, the current is reduced, and as the length of the arc is reduced the current increases. It, therefore, the speed of either of the pumps 28 or 29, or both, may be made torespond proportionately and in the same sense; as; the current varigenerally designated 44, having an armature 45 and a field 41, is coupled directly to the second motor 29. Leads 48 and 49 are connected to a suitable constant source of direct current, the field windings 43 and 41 being connected thereto inlparallel in such manner as to insure constant field energization. A current transformer 5| is coupled inductively to the arc circuit, as to the electrode conductor l2, the output being connected to a suitable rectifier 52, such as the dry plate type. The output of the rectifier 52 is connected in series with the armature 42 across the constant current leads 48 and 49 in such manner that the voltage output of the rectifier 52 will be added to the voltage in the leads and 48. A variable resistance 53 may be employed to regualthough it will be understood that other-means of current regulation on either the direct current or alternating current side of the rectifier 52 may be substituted therefor." The values of the maximum voltage output of the rectifier 52 during operation and of the line 48-49 should be such that when each is at a' maximum value' the sum of the two is equal to the maximum voltage for which the motor" isdesignedi- The charge 38 in the shell 39 is suitably grounded as at 5'4. Apotential' transformer 58 has a primary winding 51 connected between the electrode conductor l2and the ground 54. The output of the secondary 58 of the potential transformer 58 is passed through-"a rectifier 59,the output "of which is passed through the armature 46 of the-motor 44. A variableresistance sunn be placed in series in-the output circuit-of the rectifier59 -to control the current passing through the armature 48.

In operation, it will be noted'that the motors 44 and 4| will revolve continuously in the same direction and will vary only in speed. In each instance, the fields are of constant strength and virtually the entire speed regulation of these motorsis obtained by varying the armature current. In the case of the motor 4|, the armature 42 will always be supplied with a predetermined minimum direct current voltage, this voltage being supplemented by the output of the rectifier 52 when the transformer 5| is energized. The

therefore-vary between zero and some maximum value. As the electrode- |1 approaches the charge 38, assuming an arc to exist, the current s'flowing through the bus bars -l2 will increase, 6

thereby increasing the speed of the motor 4| and increasing the discharging volume of the pump 28. Also, as the arc shortens, the current flow-- ing through the primary winding 51 of the transformer 56 is reducedJthus reducing the amount of current flowing through the armature 48 or the motor 44 which, of course, reduces the speed of this motor.- ,It will thus be seen that by a change in the intensity of current, the speed of the first motor is increased and the speed of the second motor lsrdecreased these-variations :act-

speed of both motors, since rent transformer 51, I a of the field throu h thetran r ares, thus iincre sinst eispeed th length oi he are... "I

imum current into the armature :46 since the output or the tran former 56 willnot dro to zero in orm operation.-

It is not necessary ,in every instance to very a veris i he speed of either can b ma e :to Produce the desired electrode movement at a rate proportional to the iat o -e c rode current r m a p determined, or Selected normal. As illustrated in ig. 3 th m tor 4! is onnect d dire t y t th constant direct current supply lines 48-749. The ,iield 43,:and the armature 42 being connected in sh nt or parallel as ndic ted in such ma ner as to drive the motor 4|, at aeonstantspeed. A current transformer conductively coupled to the conductor I2 energizes a rectifier 52. The output of the rectifier 52 is placed in series with the constant voltage supply 481-49 and with the field n n 7 of m tor 44 in such manner that the field winding 41 will at no time fall to a voltage below that suppliedtrom the lines 48-49,. The potential transformer 56, asin the previous illustration, energizesthe rectifier 59,, which in turn energizes the armature 46 of the motor 4 4.

In this instance, it will be noted that the mor 1, ein c nnected insh t, lo ate a a substantially constant speed su-pplying hyd oulic fluid at a predetermined rate-t the cyli der22. The speed of the second motor 44, which removes qu d f m e cy in e ov t es inversely with the current flowing through the conduc or 2 in theo d tor fall. the output of the recti .52-w l1 be redu ed. Sin e. h ou u o e eotifier 52 is in series with the directcurrent supthereby supplementing such volta e, the voltage normally su plied to he field win ns 4' Will be reduc d, h increas n t e speed o t e m or 44. Sinc he re uc ion n cu e nthe conducto 2 i theresu tot increased voltage drop across the arc of the elec- .t ode I1, t e ou pu of e t ansformer 6 and t r c fie 9 w l in ase. th reby increasin t cu e t to, the arm ture .5 Th s also is efl' ctive in incr as ng theso ed oi the mo or- 44. bo h si nal ope atin of course. such man.- ner as to r du e the vo teee drop ocros thearc b l werin the e ect ode. th reby rest rin the current in the conductor l2 to normal,

- he cont ol prin iple il u trated inv Fig. scan ce-ap ed s milarly to the motor M. as llust a ed n i 4.v In this ins ance. th motor 4 is conne e i onventi nal manner to a sub.- stantially constant source of direct current,; the held 41 bein co nected. in shunt with the arma- 1 ture 46. The substantially con tant direct current source 48-49 is, as previously connected in series with the rectifier described, 52 in such manner that the voltages are additive. The rectifier 52 is energized by the current transformer 5| as previously described, the outputof the rectifier 52 being passed of the first motor 4|. The output or the potential transformer 56 is, as before, rectified by the rectifier 59, the output of which energizes the field winding 48 of the motor 4!, In this instance an increase in I ei a i f h r atu 5 2 through the u and decrease the strength oijthemotor For xam l shou d the cur en through the armature 42 current i cre se the e I: and length,- 7

simultane usly increase and decrease th rel ive speed o he motors as r q r d, When eniploying .the type of motor herein described. namely, a shunt wounddirect current motor, the output of the current transformer 5| maybe connectedto either the armature of the first motor, thetfield of the down motor. or.both. The output of the potentialtransformer may similar.- ly be connected to the armature o the secon motor, the field of the first motor, or both. In add ti n, both may be used simultaneously s i lustrated in Fig. 5. For convenience of illustration, two separate sources of substantia ly constant direct current 48- 49 and 4,8 49 ,a,re lillust at d- Asv in pr vious xamples. curren transformer 51 is inductively coupled to the'conuct rl2 d e g s. rectifier 5. 1 wh h in urn energizes the armature 42 of the first motor. The armature 42 isconnected in series with a source of direct current lie-49 the field .41 of the second motor also being placed inseries in this circuit. It will be noted that the field 4! is at a l im s. n rgized. h tential tra sfo mer, 56 n r z s. rectifier 59,- .th output of which is onnec ed with thearmature 46 of the motor 44. The armature t6 is in series with the direct current source-48s-49 and with field 43 of the motor 4|.

I t is instance, a .decreose n cu rent inthe conductor 12 reduces the speed of the first motor its speed. The increase intheoutput of rectifier- 59 also increases the stren th of the field of thevmotor 4 I, which iurther reduces the speed of this motor. The length clothe arc is therefore rapidly reduced and. the current .flow inthe conducto l2 res red t normal.

, In ch nstan e, i wi l. he noted that an auto ransformer or sim lar re ulatin dev ce may b employed to re u ate the'ou nut of the trans.- fo m r 5| and 56 accordancerwith conventionel P a e .It ill also. bemiderstood that -,invention.

he v u h draulic. mechanical and electrical arrangements hereinbefore described are applicable to a single phase of a three phase furniec the other phases being substantial duplicat s f ha d s r d- It w ll also be appreciated that by properly selecting motor voltages in comparison with arc circuit voltages the transformer 56 may in some instances be eliminated and the alternating current drawn from the electrode circuit rectified directly by the rectifier 59. This usually isno't considered desirable, however, Since the various motor windings. ener ized by the ou put of the rectifier 58 must be const u ted to carry relatively hi h ampera e, low voltage curren a d e elthere ore usual y more expensiv t ll. be app rentrmm'study of the control herein. desc ed that ,h h motors may ha e the ent maximum operating speeds, the pumps may be of the same size. In either event and regardlessof whether the discharge volumes or one or both pumps vary, there mustbe a point at which the'discharge volume of the first pump equals the discharge volume of the second pump, this being the condition obtained when the arc is operating stably in the desired arc length.

I claim:

1. In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which includes first and second rotary positive displacement hydraulic pumps driven respectively by first and second unidirectional motors, the first pump having an outlet communicating with said servomotor and with the second pump inlet, a liquid reservoir communicating with the first pump inlet and the second pump outlet, and means responsive to variation in the arcing gap of the electrode for varying the speed of one of the motors through a range including an'intermediate speed at which the output of the first pump equals the output of the second pump,'said last mentioned means including a transformer inductively coupled to said electrode conductor and to said motor.

2. In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which includes first and second rotary positive displace- "ment hydraulic pumps driven respectively by first and'second unidirectional motors, the first pump having'an outlet communicating with said servo- 'motor and with the second pump inlet, a liquid reservoir communicating with the first pump inlet and the second pump inlet, and means responsive to variation in the arcing gap of the electrode for varyingthe speed of the first motor through a speed range including an intermediate speed at which the discharge volumes of the first and second pumps are equal, said means including a current transformer inductively coupled to the electrode conductor means and connected to said first motor, therebyvarying the speed of said first motor directly with and proportionately to the current fiow in the conductor means. r

3. In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which includes first and second rotary positive displacement hydraulic pumps driven respectively by first and second unidirectional motors, the first pump having an outlet communicating with said servomotor and with the second pump inlet, a liquid reservoir communicating with the first pump inlet and the second pump outlet, and means responsive to variation in the arcing gap of the electrode for varying the speed of the second motor through a speed range including .an intermediate speed at which the discharge volumes of thefirst and second pumps are equal,

said means. including ,the"currentitransformer inductively coupled to the electrode conductor "is ductor' means, means rectifying the output of the 8 means and connected to said second motor thereby varying the speed of said motor inversely and proportionately to the current fiow in the electrode conductor means.

4. In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which includesfirst and second rotary positive displacement hydraulic pumps driven respectively by first and second unidirectional motors, the first pump having an outlet communicating with said servomotor and with the second pump inlet, a liquid reservoir communicating with the first pump inlet and the second pump outlet, and means responsive to variation in the arcing gap of the electrode for simultaneously varying the speed of both of said motors through a range including an intermediate point at which the discharge of the first pump is equal to the discharge of the second pumpjsaid means including the current transformer inductively coupled to the electrode conductor means and connected to both the first and second motors.

5. In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which includes first and second rotary positive displacement hydraulic pumps each having an inlet and an outlet, the outlet of the first pump communicating with said servomotor means and with the second pump inlet, a reservoir communicating with the first pump inlet and the second pump outlet, firstand second motors coupled respectively to said first and second pumps, said first motor being a direct current shunt wound motor ductor means, means rectifying the output of the current transformer and means supplying such rectified current to the armature winding of said first motor.

6. "In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which includes first and second rotary positive displacement hydraulic pumps each having an inlet and an outlet, the outlet of the first pump communieating with said servomotor means and with the second pump inlet, a reservoir communicating with the first pump inlet and the second pump outlet, first and second motors coupled respectively to said first and second pumps, said second motor being a direct current shunt wound motor having a field winding and an armature winding and means responsive to variation of the arcing gap for varying the speed of the second motor through a range including an intermediate speed at which the discharge volumes of the two pumps are equal, said means including a current transiormer inductively coupledto the electrodev concurrent 'transionner and rectified current; tdthefieid winding of said second motor. v I

7. Inv an electric: arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for v y; ing the electrode position, the improvement which includes first and second rotary positive displacement hydraulic pumps each having an inlet and outlet, the outlet of the first pump communi'cati ng with said servomotor means and with the second pump inlet, a reservoir communicating with the first 'puinpinlct and the second pump outlet, first and second motors coupled respectively to said first and second pumps, both said first and second motors being direct current shunt wound motors, each having field windings and an armature winding, and means responsive to variation of the arcing gap for varying the speeds of said motors through ranges including an intermediate point at which the discharge volumes of the two pumps are equal, said means including a current transformer inductively coupled to the electrode conductor means, means rectifying the output of the current transformer and means supplying such rectified current to both the armature winding of the first motor and to the field winding of the second motor.

8. The structure defined in claim which includes means maintaining a constant minimum energization of the armature of the first motor.

9. The structure defined in claim 6, which includes means for maintaining a constant minimum energization of the field winding of the second motor.

10. In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which includes first and second rotary positive displacement hydraulic pumps, each having an inlet and an outlet, the outlet of the first pump communicating with said servomotor means and with the second pump inlet, a reservoir communicating with the first pump inlet and the second pump outlet, first and second motors coupled respectively to said first and second pumps, both of said motors having direct current shunt field and armature windings and means proportionately responsive to variation in the arcing gap of the electrode for varying the speeds of the motors inversely through a range including a point at which the discharge volume of the first pump equals the discharge volume of the second pump, said last mentioned means including a current transformer inductively coupled to the electrode conductor means, means rectifying the output of the current transformer, means transmitting such rectifying current to the armature of the first motor and to the field of the second motor, a potential transformer connected from the electrode conductor means to ground, means for rectifying the output of said potential transformer, and means transmitting the rectified output of said potential transformer to the armature of the second motor and to the field of the first motor.

11. The structure in claim which includes means maintaining a predetermined minimum mass; supplying such.

. 10 degree: of energization on the fields of both motors.

' 12. in an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying elem tric current to the electrode and liquid servo motor means coupled to the electrode for varying the electrode' position, the improvement which includes" first and second rotary positive displacement hydraulic pumps, each having an inlet and an outlet, the outlet of the first pump communic'ating with the second pumpinlet, a reservoir communicating with the" first pump inlet arid the second ump outlet, first and second motors coupled respectively to' said first andse'cond pumps, both of said motors having direct current shunt field and armature windings and means proportionately responsive to variation in the arcing gap of the electrode for varying inversely the speeds of the motors through a range including a point at which the discharge volume of the first pump equals the discharge volume of the second pump, means for constantly energizing the fields of the first and second motors, a current transformer inductively coupled to the electrode conductor means, means rectifying the output of the current transformer, means transmitting such rectified output to the armature of the first motor, a potential transformer connected from the electrode conductor to the ground, means rectifying the output of said potential transformer and means transmitting the rectified output of the potential transformer to the armature of the second motor.

13, In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which includes first and second rotary positive displacement hydraulic pumps, each having an inlet and an outlet, the outlet of the first pump communicating with said servomotor means and with the second pump inlet, a reservoir communicating with the first pump inlet and the second pump outlet, first and second motors coupled respectively to said first and second pumps, means for driving the first motor at a substantially constant speed, the second motor being direct current shunt wound motor having a field Winding and an armature winding, and means proportionately responsive to variation in the arcing gap of the electrode for varying the speed of the second motor through a range including a speed at which the discharge volume of the first pump equals the discharge volume of the second pump, said means including a current transformer inductively coupled to the electrode conductor means, means rectifying the output of the current transformer, means transmitting such rectified output to said second motor field, means maintaining a predetermined minimum degree of energization on said second motor field, a potential transformer connected from the electrode conductor to ground, means rectifying the output of said potential transformer and means transmitting the rectified output of said potential transformer to the armature of said second motor.

14. In an electric arc melting furnace of the type having an electrode movable for varying the arcing gap, conductor means for supplying electric current to the electrode and liquid servomotor means coupled to the electrode for varying the electrode position, the improvement which said servomotor means and with 11 includes first and second rotary positive displacement hydraulic pumps each having an inlet and an outlet, the outlet of the first pump communicating with said servomotor means and with the second pump inlet, a reservoir communicating with the first pump inlet and the second pump outlet, first and second motors coupled respectively to said first and second pumps, means for driving said second motor at a substantially constant speed, said first motor having a direct current shunt field and direct current armature and means proportionately responsive to variation in the arcing gap of the electrode for varying the speed of said first motor through a range including a point at which the discharge volume of the two pumps are equal, said means including a current transformer inductively coupled to. the electrode conductor means, means rectifying the output of the transformer, means transmitting such rectified output to the armature of said first motor. a potential transformer connected from electrode conductor to ground, means rectifying the output of such potential transformer, and means transmitting the rectified output of said potential transformer to the field of the first motor, and means for maintaining a predetermined degree of energization on the field winding of the first motor.

No references cited. 

